1. Field of the Invention
The present invention relates generally to a two-stage injection unit for an injection molding machine and, more particularly, to a check ring assembly used in conjunction with the plunger of the melt accumulator in a two-stage injection unit.
2. Description of the Related Art
The injection unit of an injection molding machine provides essentially two functions during the course of a normal cycle of operation; namely, injection and extruder. In a standard reciprocating screw injection molding machine, the extruder function is accomplished when the screw is rotated, gradually moving plastic melt toward the forward end of the screw, thereby creating a pressure or force to move the screw rearward to its pre-injection position as the melt accumulates. When a sufficient amount of material is accumulated ("a shot"), the screw is moved rapidly forward (without rotation) to inject the melt straight into the mold, thus performing the injection function.
The injection unit of a molding machine can also be designed as a "two-stage" system where the extruder and injection functions are performed by separate machine elements. In a two-stage injection system, the extruder or plasticizing function is still performed by a feed screw in a heated barrel, but all or part of the plastic melt is diverted into an "accumulator" rather than being conveyed directly to the mold. The accumulator is subsequently operated to perform or, at least, assist in performing the injection function. The accumulator is essentially a variable volume reservoir comprising a tubular barrel and a reciprocating plunger. The relative size of the barrel and plunger, as well as the stroke of the plunger, will vary according to the quantity of melt required to fill the mold. The advantages of a two-stage injection unit include more uniform plastication of material, reduced wear on the screw and barrel, and the potential for higher injection pressures.
In the prior art, two stage injection plungers have operated at very small running clearances between the outer diameter of the plunger head and the inner diameter of the bore of the barrel. This type of construction minimized the amount of material that flowed over the plunger during injection. (Plastic melt flowing to the "back" side of the plunger causes a major housekeeping nuisance and becomes a source of costly material scrap.) The close running clearance also improved injection efficiency because a higher percentage of injected material went into the mold instead of over the plunger as injection pressure rose to a higher level, as is required to fill thin wall parts. Since plunger strokes were very short (usually less than three times the plunger diameter) in early prior art units, the alignment between the plunger shaft that transmitted injection force and the plunger head could be maintained without encountering significant problems of galling or pick-up between the barrel and the plunger head.
Recent developments in two-stage injection have recognized the value of significantly increasing the length of stroke of the plunger relative to the diameter of the plunger head. More specifically, since the diameter of the plunger head determines the load carrying requirements for the mechanism that drives the plunger, larger shot capacities and greater shot accuracy can be accomplished with the two-stage design by providing increased length of stroke at relatively small plunger diameters. However, with the longer strokes, the close running clearance between the plunger and barrel used in the prior art is not practical, since it renders the assembly prone to the galling and pick-up problems noted above. If the clearance is increased to avoid these problems, the leakage past the plunger during injection increases significantly, causing the housekeeping and waste problems mentioned previously. The increased clearance also increases the likelihood that material that previously flowed past the plunger will contaminate the new melt that enters the accumulator during the filling process.